Short answer: Feel free to send simultaneous submissions unless individual guidelines indicate otherwise.
A simultaneous submission is where you send work to multiple publications at once. This used to be really frowned upon in the days of snail mail as it wasted everybody’s time at best; at worst, postal delivery times meant wires easily got crossed.
Nowadays, it is much more common for publications to allow or even encourage simultaneous submissions. Email and digital submission means you can quickly let an editor know if your work has been accepted elsewhere, often before it’s even been seen at the publication from which you’re withdrawing. On rare occasions, an editor might miss out on your work because they’re still deliberating; that’s just the way it goes. It’s also understood that it’s not fair to expect a writer to only submit to one outlet at a time and then wait eight months for a reply before submitting somewhere else.
Where publications prohibit simultaneous submissions, they usually offer a quick response in return for exclusive consideration. Threepenny Review, for example, is renowned for having one of the quickest reply rates in the industry—often within 24 hours. The exception is contests, where your entry might go through multiple stages of review before landing on a judge’s desk. In this case, the trade-off for taking your best work out of the general submission cycle is a high reward in terms of either monetary prize or recognition if your piece wins the contest.
News and media outlets often request exclusive submissions because when they’re interested in a piece of content, they typically accept it immediately and rush it into print. You can quickly get on an editor’s bad side by submitting an article to rival publications simultaneously. You also run the risk of someone taking your idea and writing a new article around it if you withdraw your piece to publish somewhere else. It doesn’t happen often, but there’s no way to prove when it does.
— Nicole Melanson